Generally the invention is concerned with leak detection systems and methods of the type disclosed in the present applicant's U.S. Pat. No. 4,649,739 and pending application, U.S. Ser. No. 014,812 filed Feb. 13, 1987, which related to the detection of leaks in tanks such as the underground tanks employed for storing gasoline at gasoline service stations. Newer and better methods of detecting such leaks are continuously sought in view of the very rigid requirements in many states today regarding the permissible leakage from storage tanks holding hazardous liquids. Tanks at such service stations may typically be 26 feet long, cylindrical tanks having a diameter of 8 feet, and holding perhaps 10,000 gallons of gasoline. It is normal for approved testing personnel using approved leak detection equipment to periodically check the fuel leakage from such storage tanks to be certain that leakage is within the very negligible maximum leakage limits permitted. A number of methods have been proposed including those described in the aforementioned patent and application wherein the tanks are filled incompletely such that there is a space left in the tank above the liquid level in the tank which is filled with a gas which maintains a predetermined pressure on the liquid in the tank from one end of the tank to the other. In the previously mentioned patent, a bubbling tube was inserted into the liquid in the tank to a predetermined depth and the resistance to bubbling was balanced as a reference pressure against the static pressure of the gas in the space above the liquid. Any differential variation was indicated and recorded.